linux-kernel - Re: [PATCH 3/3] objtool: optimize add_dead

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Date:   Tue, 21 Apr 2020 15:02:58 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] objtool: optimize add_dead_ends for split sections

On Tue, Apr 21, 2020 at 03:13:05PM -0500, Josh Poimboeuf wrote:
> Also, the unreachable annotation at the end of a section is really an
> edge case.  I'm sort of wondering if there's a way to accomplish the
> same thing without storing the last_insn.
> 
> For example, I wonder if we could use find_insn() for some bytes at the
> end of the section.  Most of the time I _think_ there will be a two-byte
> UD2 instruction there anyway.  So maybe we could do something like:
> 
> 	for (offset = rela->sym->sec->len - 1;
> 	     offset > rela->sym->sec->len - 10;
> 	     offset --) {
> 
> 	     insn = find_insn(file, rela->sym->sec, offset);
> 	     if (insn)
> 	     	break;
> 	}
> 
> It's kind of ugly, but then we could maybe avoid the need for the
> last_insn thing.

Sure, that looks fine. I tested this and it looks like the performance
is roughly the same. I'll send v2 shortly.

Sami